The invention relates to a process for isolating pure substances from mixtures of high-boiling air- and/or temperature-sensitive substances which require a high separation efficiency by rectification under medium vacuum, in particular a process for the rectification of crude vitamin E acetate, for the purpose of purifying from lower-boiling and higher-boiling impurities, and to columns suitable for this process.
The workup of product mixtures by distillation generally affords the best results in the case of countercurrent distillation (also called rectification), ie. a specific distillation process with a countercurrent of reflex flowing downwards and vapor flowing upwards in rectification columns. The rectification columns normally used for this purpose are such that the mixture to be separated is introduced into the middle part of the column, and the vapor on its way through the column from the bottom to the top becomes enriched in more volatile components while the reflux from top to bottom becomes enriched in less volatile components. Mass and heat transport is intensified by elements fitted inside the column, such as column plates or packings, which ensure a sufficient contact time of the phases and a sufficiently large phase boundary area. However, these column internals, together with the reflux flowing downwards, result in a resistance in the column which is called the pressure drop. The pressure drop in a column depends not only on the nature and amount of the compounds to be rectified but also very strongly on the nature of the column internals.
Generally used for the fractionation of high-boiling substance mixtures which require a high separation efficiency are rectification columns which have packings which are built up systematically in regular geometry and have defined areas for the countercurrent phases to pass through, because packings with a regular structure are distinguished in comparison with random packings by the possibility of higer flow rates and a better separation effect, and have a lower specific pressure drop and a smaller required packing volume and thus also a smaller necessary mass and heat exchanging height. They are therefore used in all vacuum rectifications in which, because of the temperature-sensitivity of the mixture to be separated, it is particularly important to limit the pressure drop in the column.
Particularly suitable column packings are metal cloth packings of the BX and CY types supplied by Sulzer (cf. Sulzer company publication xe2x80x9cTrennkolonnen fxc3xcr Distillation and Absorptionxe2x80x9d) and metal cloth packings with a similar effect supplied by other companies such as Montz GmbH.
A diagrammatic representation of such columns is to be found, for example, on page 103 of the textbook xe2x80x9cThermische Trennverfahrenxe2x80x9d by Klaus Sattler, VCH Verlagsges.mbH, Weinheim (FRG), 1988. Concerning further details of the rectification of substance mixtures, we refer to this textbook by Klaus Sattler, pages 101-225, in particular 120-160 and 199-214.
The highest product temperature occurs in the bottom of a column. Apart from the overhead pressure, it is closely determined by the pressure drop from the column internals derived from the required separation efficiency. In rectification columns, the bottom temperature is not significantly affected by reducing the overhead pressure to less than 0.5 mbar.
The thermal stressability of many high-boiling mixtures is so low that, despite the use of the described metal cloth packings with ordered structure and overhead pressures in the column of only required for the necessary separation efficiency would result in bottom temperatures which are above the decomposition range for the compounds to be separated. This is why to date the fractionation of such mixtures by distillation has generally been carried out in the high vacuum range (about 10xe2x88x921 to 10xe2x88x925 mbar), ie. short-path distillations or molecular distillations are used. However, in the case of mixtures with low relative volatilities it is possible to obtain high purities only with low yields in these distillations.
One example of a mixture of high-boiling and highly air- and/or temperature-sensitive substances which require a high separation efficiency is synthetic vitamin E acetate (VEA) which is prepared industrially by reacting trimethylhydroquinone with phytol or isophytol and subsequently esterifying with acetic anhydride, and still contains small amounts of colored lower-boiling and higher-boiling impurities. Since VEA is increasingly being used in the human diet and for health prophylaxis, the demands on the purity of this product are increasing. Rectification, which is very advantageous in general for purification of products on an industrial scale, is very difficult with VEA owing to its high boiling point together with its instability at higher temperatures. This is why to date essentially distillations under high vacuum or even molecular distillations have been carried out in order to be able to distill VEA at the lowest possible temperatures.
Despite the use of high vacuum (10xe2x88x921 to 10xe2x88x925 mbar), the purities obtained in the prior art are generally only 97.3% (cf. DE 2 743 920), 98% (cf. DE 42 08 477 and JP-B-58 011 869), 98.5% (cf. U.S. Pat. No. 3,459,773) or 98.5 to 99% (cf. DE 2 160 103). Purities above 99% have been obtained only by molecular distillation, namely purities of 99.3% according to JP-A 51/14671 and 99.5% according to JP-A-62/226976, although it should be noted that the products obtained in this way would presumably show lower purities on investigation with the more accurate analytical methods in use now and with purer comparison substances. In addition, the yields which can be obtained in such distillations are in each case rather low.
However, since distillations under high vacuum, but especially molecular distillations, while giving high purities not only have the disadvantage of low distillation yields but are also extremely costly in terms of the capital costs and in terms of the operating costs, it was object of the invention to develop a process for the separation by distillation of high-boiling air- and/or temperature-sensitive substances which require a high separation efficiency, in which high-vacuum distillation or molecular distillation is unnecessary, ie. a process in which an overhead pressure of from 0.1 to 2 mbar is sufficient and thus considerably less costly pressure-reducing processes are necessary and a high distillation yield is obtained.
It was an object of the invention in particular to find a process for the final purification of VEA by distillation able to result in colorless VEA with a primary of 99% or more and good distillation yields even by rectification under medium vacuum in columns containing metal cloth packings with ordered structure.
The invention thus relates to a process for isolating pure substances from mixtures of high-boiling air- and/or temperature-sensitive substances which require a high separation efficiency by rectification under medium vacuum in columns containing metal cloth packings with ordered structure, which comprises carrying out the rectification in a mass transfer column in which
a) the liquid distribution is undertaken with channel distributors with 500 or more drip points/m2, preferably 900 to 1200 drip points/m2,
b) in which the channels of the distributors are arranged at an angle of about 90xc2x0 to the cloth layers of the packing elements located immediately below the distributor,
c) in which 2 or more packing elements which have a height of from 20 to 100 mm and whose cloth layers are in each case rotated by 90xc2x0 with respect to one another are located immediately below the liquid distributors,
d) the column is designed so that virtually no heat exchange through the column wall can take place during the rectification, and
e) for air-sensitive substances, the column is designed so that it is possible to operate virtually with exclusion of air.
The process according to the invention takes place particularly advantageously when the cloth packings used are such that the angle of inclination of the serration of the individual cloth layers of the packing to the column axis is as small as possible in order to minimize the specific pressure drop of the packing.